We will continue studies on the relationship of testicular androgen levels and spermatogenesis and, in particular, address the following three issues: (1) Pituitary control of the transport of androgen into the seminiferous tubules. It has been suggested that FSH affects the transfer of testosterone (T) from the interstitial to the tubular compartment of the testis. To examine this possibility, and to determine whether this action of FSH can influence spermatogenesis, immature and adult rats will be hypophysectomized and treated with T, dihydrotestosterone (DHT), or pregnenolone with and without concomitant administration of FSH. After various periods of treatment, we will measure T and DHT levels in the rate testis fluid (RTF), in testicular and in tubular homogenates, and evaluate spermatogenesis by quantitative histological analysis. (2) Changes in testicular androgen levels during photoperiod-induced alterations of testicular activity in the hamster. In hamsters, 2 months of exposure to a short photoperiod causes a drastic decline in testicular weight and spermatogenesis and a significant, but relatively modest, reduction in peripheral T levels. Return of these animals to a long (stimulatory) photoperiod causes testicular recrudescence. To determine whether the photoperiod-related changes in the spermatogenic activity are caused by changes in the androgen levels in the seminiferous tubules, we will examine T and DHT levels in RTF, peripheral blood, testicular and tubular homogenates at various time intervals after changing the photoperiod. (3) Influence of the thyroid on the androgen levels in RTF. Thyroid disease is often associated with subfertility. To determine whether changes in the androgen levels in the immediate milieu of dividing and differentiating germinal cells may be associated with alterations of thyroid status, we will examine T and DHT levels in the RTF of normal, thyroidectomized, thyroxine-treated and experimentally hypothyroid rats.